Category Archives: Lighting

A GaN LED, coated with a “factory-roof” pattern modeled off the fireflies’ scales. The bio-inspired LED coating increased light extraction by more than 50 percent. (Credit: Nicolas Andr)

Jan. 8, 2013 — The nighttime twinkling of fireflies has inspired scientists to modify a light-emitting diode (LED) so it is more than one and a half times as efficient as the original.

Researchers from Belgium, France, and Canada studied the internal structure of firefly lanterns, the organs on the bioluminescent insects’ abdomens that flash to attract mates. The scientists identified an unexpected pattern of jagged scales that enhanced the lanterns’ glow, and applied that knowledge to LED design to create an LED overlayer that mimicked the natural structure. The overlayer, which increased LED light extraction by up to 55 percent, could be easily tailored to existing diode designs to help humans light up the night while using less energy.

The work is published in a pair of papers today in the Optical Society’s (OSA) open-access journal Optics Express.

“The most important aspect of this work is that it shows how much we can learn by carefully observing nature,” says Annick Bay, a Ph.D. student at the University of Namur in Belgium who studies natural photonic structures, including beetle scales and butterfly wings. When her advisor, Jean Pol Vigneron, visited Central America to conduct field work on the Panamanian tortoise beetle (Charidotella egregia), he also noticed clouds of twinkling fireflies and brought some specimens back to the lab to examine in more detail.

Fireflies create light through a chemical reaction that takes place in specialized cells called photocytes. The light is emitted through a part of the insect’s exoskeleton called the cuticle. Light travels through the cuticle more slowly than it travels through air, and the mismatch means a proportion of the light is reflected back into the lantern, dimming the glow. The unique surface geometry of some fireflies’ cuticles, however, can help minimize internal reflections, meaning more light escapes to reach the eyes of potential firefly suitors.

In Optics Express papers, Bay, Vigneron, and colleagues first describe the intricate structures they saw when they examined firefly lanterns and then present how the same features could enhance LED design. Using scanning electron microscopes, the researchers identified structures such as nanoscale ribs and larger, misfit scales, on the fireflies’ cuticles. When the researchers used computer simulations to model how the structures affected light transmission they found that the sharp edges of the jagged, misfit scales let out the most light. The finding was confirmed experimentally when the researchers observed the edges glowing the brightest when the cuticle was illuminated from below.

“We refer to the edge structures as having a factory roof shape,” says Bay. “The tips of the scales protrude and have a tilted slope, like a factory roof.” The protrusions repeat approximately every 10 micrometers, with a height of approximately 3 micrometers. “In the beginning we thought smaller nanoscale structures would be most important, but surprisingly in the end we found the structure that was the most effective in improving light extraction was this big-scale structure,” says Bay.

Human-made light-emitting devices like LEDs face the same internal reflection problems as fireflies’ lanterns and Bay and her colleagues thought a factory roof-shaped coating could make LEDs brighter. In the second Optics Express paper published today, which is included in the Energy Express section of the journal, the researchers describe the method they used to create a jagged overlayer on top of a standard gallium nitride LED. Nicolas André, a postdoctoral researcher at the University of Sherbrooke in Canada, deposited a layer of light-sensitive material on top of the LEDs and then exposed sections with a laser to create the triangular factory-roof profile. Since the LEDs were made from a material that slowed light even more than the fireflies’ cuticle, the scientists adjusted the dimensions of the protrusions to a height and width of 5 micrometers to maximize the light extraction.

“What’s nice about our technique is that it’s an easy process and we don’t have to create new LEDs,” says Bay. “With a few more steps we can coat and laser pattern an existing LED.”

Other research groups have studied the photonic structures in firefly lanterns as well, and have even mimicked some of the structures to enhance light extraction in LEDs, but their work focused on nanoscale features. The Belgium-led team is the first to identify micrometer-scale photonic features, which are larger than the wavelength of visible light, but which surprisingly improved light extraction better than the smaller nanoscale features. The factory roof coating that the researchers tested increased light extraction by more than 50 percent, a significantly higher percentage than other biomimicry approaches have achieved to date. The researchers speculate that, with achievable modifications to current manufacturing techniques, it should be possible to apply these novel design enhancements to current LED production within the next few years.

The firefly specimens that served as the inspiration for the effective new LED coating came from the genus Photuris, which is commonly found in Latin America and the United States. Bay says she has also examined the lanterns of a particularly hardy species of firefly found on the Caribbean island of Guadeloupe that did not have the factory roof structure on the outer layer. She notes that she and her colleagues will continue to explore the great diversity of the natural world, searching for new sources of knowledge and inspiration. “The Photuris fireflies are very effective light emitters, but I am quite sure that there are other species that are even more effective,” says Bay. “This work is not over.”

This illustration (not to scale) simulates the process by which an incoming complex wave can be identified and transmitted to a photodetector. (Credit: Image courtesy of Patrice Genevet)

Jan. 8, 2013 — At a time when communication networks are scrambling for ways to transmit more data over limited bandwidth, a type of twisted light wave is gaining new attention. Called an optical vortex or vortex beam, this complex beam resembles a corkscrew, with waves that rotate as they travel.

Now, applied physicists at the Harvard School of Engineering and Applied Sciences (SEAS) have created a new device that enables a conventional optical detector (which would normally only measure the light’s intensity) to pick up on that rotation.

The device, described in the journal Nature Communications, has the potential to add capacity to future optical communication networks.

“Sophisticated optical detectors for vortex beams have been developed before, but they have always been complex, expensive, and bulky,” says principal investigator Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS.

In contrast, the new device simply adds a metallic pattern to the window of a commercially available, low-cost photodetector. Each pattern is designed to couple with a particular type of incoming vortex beam by matching its orbital angular momentum — the number of twists per wavelength in an optical vortex.

Sensitive to the beam’s “twistiness,” this new detector can effectively distinguish between different types of vortex beams. Existing communications systems maximize bandwidth by sending many messages simultaneously, each a fraction of a wavelength apart; this is known as wavelength division multiplexing. Vortex beams can add an additional level of multiplexing and therefore should expand the capacity of these systems.

“In recent years, researchers have come to realize that there is a limit to the information transfer rate of about 100 terabits per second per fiber for communication systems that use wavelength division multiplexing to increase the capacity of single-mode optical fibers,” explains Capasso. “In the future, this capacity could be greatly increased by using vortex beams transmitted on special multicore or multimode fibers. For a transmission system based on this ‘spatial division multiplexing’ to provide the extra capacity, special detectors capable of sorting out the type of vortex transmitted will be essential.”

The new detector is able to tell one type of vortex beam from another due to its precise nanoscale patterning. When a vortex beam with the correct number of coils per wavelength strikes the gold plating on the detector’s surface, it encounters a holographic interference pattern that has been etched into the gold. This nanoscale patterning allows the light to excite the metal’s electrons in exactly the right way to produce a focused electromagnetic wave, known as a surface plasmon. The light component of this wave then shines through a series of perforations in the gold, and lands on the photodetector below.

If the incoming light doesn’t match the interference pattern, the plasmon beam fails to focus or converge and is blocked from reaching the detector.

Capasso’s research team has demonstrated this process using vortex beams with orbital angular momentum of −1, 0, and 1.

“In principle, an array of many different couplers and detectors could be set up to read data transmitted on a very large number of channels,” says lead author Patrice Genevet, a research associate in applied physics at SEAS. “With this approach, we transform detectors that were originally only sensitive to the intensity of light, so that they monitor the twist of the wavefronts. More than just detecting a specific twisted beam, our detectors gather additional information on the phase of the light beam.”

The device’s ability to detect and distinguish vortex beams is important for optical communications, but its capabilities may extend beyond what has been demonstrated.

“Using the same holographic approach, the same device patterned in different ways should be able to couple any type of free-space light beam into any type of surface wave,” says Genevet.

Coauthors on this work included Jiao Lin, a former postdoctoral fellow in Capasso’s lab (now at the Singapore Institute of Manufacturing Technology), and Harvard graduate student Mikhail A. Kats.

The research was supported by the U.S. Air Force Office of Scientific Research, the U.S. Intelligence Advanced Research Projects Agency, and through research fellowships from the Agency for Science, Technology, and Research in Singapore and the U.S. National Science Foundation (NSF). The researchers also benefited from facilities at Harvard’s Center for Nanoscale Systems, a member of the NSF-supported National Nanotechnology Infrastructure Network.

When scientists at the National Renewable Energy Laboratory (NREL) tried to apply their expertise in solar cell technology to build a green LED light from the ground up, they surprisingly centered the objective at their very first try. In doing so they solved a long-standing technological problem and paved the way for the large-scale employment of white LEDs for public and domestic illumination over the course of the next few years.

What’s wrong with your good ol’ tungsten bulbs, you may ask? The problem is that they produce light by incandescence, which is about the least efficient way to produce light — it wastes the majority of energy to produce useless heat, which inevitably ends up inflating your electrical bill. To a lesser extent, compact-fluorescent lights also share this inefficiency problem, which has led the U.S. Department of Energy to predict that both kinds will be phased out in the space of only four and ten years respectively, leaving LEDs virtually the only player in the market.

LED lights are unanimously regarded as a vast improvement over previous light bulbs because of their much longer lifespan and higher efficiency, which ends up saving us money in the long run, even when the higher initial cost is taken into account.

But to create a white LED, red, blue and green light need to be combined. While the first two colors have been relatively easy to manufacture, researchers have struggled to produce a green LED. The LED-based lights available today circumvent the problem by aiming the blue light at a phosphor, which then emits green light. This does produce white light, but it is still wasteful compared to a white light that makes use of three distinct, all-LED components.

NREL researcher Angelo Mascarenhas, who holds patents in solar cell technology, realized that a LED can be thought of as the reverse of a solar panel, since one takes electricity and turns it into light, while the other takes (sun)light and turns it into electricity.

Mascarenhas used the knowledge gathered by NREL when they created a world-record inverted metamorphic solar cell by combining layers of different lattice sizes to optimally capture solar energy across the visible spectrum. The researchers had already tackled the problem of how to absorb sunlight in the green spectral region, and Mascarenhas built on this knowledge to reverse the process in order to manufacture a green LED.

Absorbing green light is technically challenging because of the way the different layers of lattice that should absorb it are manufactured: if the layers don’t match up with the layer below, leaving too big a gap, the efficiency plummets to next to zero. NREL’s solution was essentially to insert extra layers of lattice that gradually bridge the gap, improving the cell’s efficiency.

Mascarenhas’s idea was to reverse the process — that is, making a current flow between appropriately spaced layers of lattice to obtain green light – and reportedly managed to produce a radiant deep green light on the very first try.

NREL is now trying to produce a fourth color to make the white light even whiter. They plan to arrange the four colors in a beehive structure, each cell being an LED of a specific color, so that the light will look white when seen from a distance.

The researchers also plan to make their LED light “intelligent,” dynamically matching the percentage of the four colors to the time of the day – for instance, increasing the blue component during daylight, or the reddish-yellow during the night.

Further predictions from the Department of Energy estimate that the move toward LEDs for both public and domestic lighting could save the U.S. as much as US$120 billion over the course of the next 20 years, as well as 246 million metric tons of carbon (approx. 5 percent of U.S. total emissions during the period 1850 through 2000) that would otherwise be released into the Earth’s atmosphere.

While the idea of using photovoltaic technology in windows to harvest sunlight for conversion to energy is not new, Smart Energy Glass (SEG) is taking a slightly different approach with a solar window that can be darkened or lightened for comfort and convenience.

The window’s opacity can be adjusted for three modes; dark, privacy, and light. Dark will harvest some light, while privacy will harvest the most. The energy is used to power the window itself and eventually lights and ventilation could be run from the energy harvested. Additionally clients can choose the color of glass and add logos if required.

SEG has obvious advantages for office cladding as in summer heat and light streaming through the window can make working conditions uncomfortable, while in winter much warmth is lost via the glass. But it could also have advantages on a smaller or domestic scale, for presentations, or in homes with sun-facing windows.

No details are yet available on the exact mechanics of the Smart Energy Glass. Peer+ says the patent pending technology is still under development and we can expect updates as pilot programs get underway this year in the Netherlands.

Marketed as the Energy Smart LED Bulb, the product should hit shelves by the end of 2010, GE says. Not only does it last 25 times longer than the bulbs consumers are accustomed to, it also substantially cuts energy use (requiring only 9 watts to emit the same 450 lumens) — presumably saving homeowners cash off their energy bills for more than a decade.

The one catch? Its price tag exceeds $40. Who will be willing to pay that much when people are used to buying a four-pack of bulbs at the grocery store for under $10? GE says it has faith that consumers will weigh potential savings against upfront cost, but this has never been a strength of the mass market.

The GE bulb is revolutionary for several other reasons. One of the reasons LEDs haven’t been widely adopted for general home and workplace lighting is because their beams tend to be focused, rather than diffuse. One of the companies working on this problem is Bridgelux, which just came out with its own screw-in LED bulb, called the Heleion. The Energy Smart LED bulb uses plastic structures wrapped around the glass to more evenly distribute the light.

GE also plans to message the new product as safer than regular bulbs. It doesn’t contain mercury, making it easier to dispose of without worrying about toxic waste or watershed contamination. And it doesn’t need to heat up to cast off light, making it less of a fire hazard than its predecessors.

The competition in the LED market is now three-fold. First, who can come up with an LED bulb to rival 60-watt incandescents, giving consumers more options? Second, who can create a screw-in LED bulb that can literally replace the bulbs we’re using now in traditional lighting fixtures? And third, who can do it at the lowest cost? Bill Watkins, CEO of Bridgelux, predicts that his company will have its interchangeable LED bulbs available for below $10 in the next year or so.

Bridgelux is also pursuing a different marketing strategy that GE might want to consider. Instead of selling LED bulbs to consumers in stores like Home Depot and Safeway, the company plans to target the people who make lighting fixtures, including architects and contractors. On top of that, it is focusing on the commercial lighting industry.

Watkins argues that companies are much more likely to adopt general LED lighting than homeowners. Why? Because companies keep more precise tallies on how much they are spending on lighting versus how much they are saving in energy and on their energy bills. A simple cost-benefit analysis will show that the LED bulbs pay for themselves over time, he says. Homeowners don’t look that far ahead.

If GE, Bridgelux, Osram Sylvania, Panasonic and others successfully infiltrate commercial lighting, they could build up a nice cushion to bring down the cost of LED lighting for the residential market — maybe within the next two years.

LED lighting company Bridgelux says out with the Edison screw-in bulb and in with the snap-in lighting module.

The Livermore, Calif.-based start-up said Wednesday it has designed a lighting module called Helieon that combines Bridgelux’s LED lights and a snap-in interconnect system made by Molex, which is based in Lisle, Ill.

The interconnect system will make it easy to install LEDs and upgrade them when more efficient or brighter lights come out, the companies said.

The Helion system, which will be available in May for $20, is aimed at lighting manufacturers that build actual lighting fixtures. The Helion will be available with a light output between 500 and 1,500 lumens, the equivalent of between 40 watts and about 100 watts for incandescent bulbs but will use significantly less electricity.

Bridgelux says that the packaging and efficiency of its lighting system is a step toward making LED lighting more cost-effective when compared with other forms of lighting. “Solid state lighting is poised to displace conventional incandescent, fluorescent and other technologies in many high-volume general lighting applications,” Bridgelux CEO Bill Watkins, who joined the company earlier this year, said in a statement .

Bridgelux has signed on some lighting fixture manufacturers to use Helieon, including architectural lighting company Focal Point. But as it goes after the general lighting market, it faces competition from a number of LED start-ups and established lighting companies.

Reinterpreting the history of lighting, designer Federico Delrosso has come up with a simple yet innovative lamp that resembles the shape of a traditional light bulb to be used as indoor as well as outdoor lighting. The “Estasi Lamp,” as hailed by the designer, is made of Technogel immersed with the LED light to illuminate in the dark. Free from all typology, the portable lamp can be employed as both a floor lamp and table lamp. “The softness of the material is reflected not only on the physicality of the lamp but also the diffusion of the light and therefore the atmosphere it sheds in the surrounding ambiance.”